WASHINGTON—The National Security Agency—which possesses only limited legal authority to spy on U.S. citizens—has built a surveillance network that covers more Americans' Internet communications than officials have publicly disclosed, current and former officials say.

The system has the capacity to reach roughly 75% of all U.S. Internet traffic in the hunt for foreign intelligence, including a wide array of communications by foreigners and Americans. In some cases, it retains the written content of emails sent between...